A crankcase lubricant is an oil used for general lubrication in an internal combustion engine where an oil sump is situated generally below the crankshaft of the engine and to which circulated oil returns. It is well known to include additives in crankcase lubricants for several purposes.
Phosphorus in the form of dihydrocarbyl dithiophosphate metal salts has been used for many years to provide lubricating oil compositions for internal combustion engines with antiwear properties. The metal may be zinc, an alkali or alkaline earth metal, or aluminium, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZDDPs) are most commonly used. However, anticipation of stricter controls on the amount of phosphorus in finished crankcase lubricants has led to the need to, at least partially, replace ZDDP in such lubricants.
The art describes phosphorus-free antiwear additives in the form of zinc dithiocarbamates, some of which are commercially-available, such as under the trade names Vanlube EZ and Vanlube AZ. Also, Palacios, J. M. Wear, 1987, 114, 41-49 and V. K. Vermia el at, Lubrication Science 16-2, Feb. 2004 (16) 195-203 report the use of zinc dithiocarbamates. A problem with use of such zinc dithiocarbamates in lubricating oil compositions is their adverse effect on the fluoroelastomer seals compatibility properties of the compositions, such seals being commonly used in piston engines.